We obtained DNA sequences coding for the nucleoprotein (NP) of an influenza A virus by reverse-transcription of vRNA using synthetic oligonucleotide primers. Terminal sequence analysis showed that the cloned gene contained a full-length copy of the virion RNA segment. The NP specific DNA was inserted into the late region of an SV40 vector and the DNA recombinant was propagated in the presence of an early SV40 ts mutant helper. Infection of African green monkey kidney cells with the recombinant produced a polypeptide immunoprecipitable with NP-specific antisera. The polypeptide product had a molecular weight of 56K daltons identical to the nucleoprotein of influenza virus as estimated on polacrylamide gels. The putative NP was detected in the nucleus of infected primate cells by an immunofluorescence assay. This nuclear localization of NP from recombinant DNA was similar to that seen during influenza virus infection suggesting the NP product may be functionally active. To further delineate the functional domains of NP as defined by several classes of monoclonal antibodies, we have recently constructed deletion mutants of the NP gene and prepared recombinants of SV40-NP. These recombinants synthesized polypeptide products representing different regions of the full-length NP molecule. Experiments are underway to characterize the cellular location of these shortened NP polypeptides and their antibody binding affinities.